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Wednesday, April 21, 2010

Constantinople's Volcanic Twilight

By 1453, triple-walled Constantinople, one of the most coveted and magnificent cities in the world, had stood watch over the Bosporus for 2100 years. For the last 1100 of these, it had been the capital of the Byzantine Empire, crown of the Eastern Christian world and an international center of wealth, beauty, power and commerce. Constantinople was named in the fourth century for the reigning Roman emperor, Constantine the Great; in antiquity the city was known as Byzantium, and today it is called Istanbul.

Also by 1453, Constantinople had been besieged many times—by Persians, Avars, Arabs (twice), Bulgars, Russians (three times) and Pechenegs. During the Byzantine era, it had been conquered only once—in 1204, by soldiers of the Fourth Crusade. The city served as the capital of the Crusaders' short-lived Latin kingdom until it was recaptured by the Byzantines in 1261.

And by 1453, the Byzantine Empire had been shrinking steadily for some 400 years, due to both internal political strife and military pressure from rival powers of both East and West. The capital city had suffered along with the rest of the empire. By 1453, Constantinople's population, once a million strong, had shrunk to a mere tenth of that. Although its historic luster had been tarnished, Constantinople, the gateway to Europe, was nevertheless an alluring military objective for the Ottoman Turks as they consolidated their hold in Asia Minor and Eastern Europe.

The Muslim Turks had first crossed into Europe a century earlier, invited by a pretender to the Byzantine throne who needed Turkish troops to enforce his claim. A decade later, in 1354, the Ottomans established a permanent presence in Europe, taking advantage of a devastating earthquake in Thrace to cross the Hellespont, occupy the ruined city of Gallipoli and rebuild it into a garrison town. From there, the Turks spread out into other parts of Thrace. In 1377, their forces defeated a large Serbian army on the Maritsa River, paving the way for future victories in Macedonia, Serbia and Greece. The second most important city of the Byzantine Empire, Thessalonica, fell to the Ottomans in 1430.

Despite several military assaults on Constantinople itself, Ottoman efforts to wrest the capital from Byzantine control proved unsuccessful. According to an ancient belief popular with the inhabitants of the city, Constantinople would fall only when the moon gave a sign.

During the Byzantine capital's waning days, in May of 1453, the ancient myth appeared to come true. As the city lay besieged by the forces of Ottoman sultan Mehmet II, the moon went into a long and dark eclipse. Constantinople's Byzantine defenders were filled with paralyzing despair; outside the walls, Ottoman troops enjoyed cautious hope.

Constantinople's final days under Byzantine rule witnessed still other unusual, seemingly apocalyptic occurrences: abnormally violent weather, lurid sunsets and sunrises, and flickering lights visible in the night sky. All were metaphysically interpreted as portents of a great change in the world order.

Today, 500 fears later, an American astronomer has proposed that a volcanic eruption in the South Pacific—half a world away from Byzantium—may have been powerful enough to darken the skies over Constantinople and to produce the other curious phenomena that coincided with the city's historic change of power. Kevin Pang, formerly of the National Aeronautics and Space Administration's Jet Propulsion Laboratory in Pasadena, California, points to the volcano Kuwae, which erupted in the New Hebrides, 1900 kilometers (1200 miles) east of Australia. Although the date of the blast is not certain, much evidence points to the year 1453.

When it erupted, Kuwae spewed out more than six times as much molten rock and ash as did the Philippines' Mount Pinatubo in 1991. When the eruption was over, Kuwae itself, once an island, was only a submarine crater 12 kilometers (7.5 miles) across.

Pang draws on calculations of astronomical orbits for part of his research, but he also reaches back into historical records to piece together how a variety of natural events, such as comet sightings or volcanic eruptions, may have affected the course of human history. His family heritage in China has enabled him to search provincial records in that country too.

The volcanic cloud from Kuwae, Pang hypothesizes, would have shrouded the earth thickly enough to darken the moon above Constantinople beyond the usual, dulled-copper appearance of a lunar eclipse. Similarly, the cloud of suspended particles could be responsible for the unseasonably cold weather, with rain and snow, and for the bizarre optical effects reported by various chroniclers, all of which are phenomena now known to be associated with volcanic eruptions. But Kuwae remains unique, says Pang, because it appears to have thrown its volcanic veil over one of the great turning points of world history.

By the lime Mehmet II's campaign to conquer Constantinople began, in the spring of 1453, the Ottomans had already reduced the ailing Byzantine Empire to fragments. Mehmet, barely 21, had succeeded his father Murat II as sultan just two years earlier. Intelligent and inquisitive, Mehmet had been an assiduous student of philosophy, science and the governmental arts. The Byzantines, however, underestimated the young sultan's talents and resolve. They failed to grasp the seriousness of his commitment, dating from the moment of his father's death, to capture Constantinople and make that city the crowning jewel of the expanding Ottoman Empire.

On April 6,1453, the Ottoman forces, under the command of Mehmet himself, set up camps outside the city's imposing triple defensive walls. Mehmet's army, which historians estimate numbered 80,000 men, vastly outnumbered the 7000 or so Greek, Venetian and Genoese troops under siege. When the Byzantine emperor Constantine XI Palaeologus refused the sultan's offer of a peaceful surrender, the Ottomans began bombarding the walls with their cannon. The Byzantines, who had no heavy guns, feverishly sought to repair the damage. For weeks the siege continued, its outcome all but assured. Although food grew scarce inside the walls, Constantinople's rulers continued to hope—in vain—for the arrival of ships from Europe bringing supplies and troops. The morale of the inhabitants declined, and there were increasing reports of people deserting the city under cover of darkness.

It was almost seven weeks into the siege, on May 22, when the lunar eclipse took place. A Venetian surgeon, Nicolò Barbaro, who was residing in the city at the time, recorded his impressions:

"At the first hour of the night, there appeared a wonderful sign in the sky, which was to tell Constantine the worthy, emperor of Constantinople, that his proud empire was to come to an end.... The moon rose, being at this time at the full...but it rose as if it were no more than a three-day moon, with only a little of it showing.... The moon stayed in this form for about four hours."

A few days later, more "signs" appeared. Just as the Byzantines were seeking divine favor with a religious procession through the city, a tremendous thunderstorm checked the supplicants' progress. Dangerous floods and pelting hail brought a quick end to the ceremony.

"Such was the unheard-of and unprecedented violence of that storm and hail [that it] certainly foreshadowed the imminent loss of all," wrote the Greek chronicler Kritovoulos of Imbros, "and...like a torrent of fiercest waters, it would carry away and annihilate everything."

The following day, a dense fog enveloped the city. Again, it was extraordinary weather for late spring in temperate Constantinople. Other chroniclers reported that observers both inside and outside the walls also witnessed a light like that of a fierce fire around the dome of Hagia Sophia, the imposing cathedral of Constantinople. Yet the building never burned. Phrantzes, a friend of the emperor and author of one of the major Byzantine accounts of the time, said that the light remained over the city for an entire night.

Accounts also tell that Mehmet had ordered his troops to light fires and torches before every tent in the Ottoman encampment. Of these illuminations, Barbaro recorded on May 26 that "the light from them was so strong that it seemed as if it were day." The fires were lit again on the two succeeding nights. Might the light from the camps have been fired by the Byzantines' imagination into something more?

According to historian Steven Runciman, "lights, too, could be seen from the walls, glimmering in the distant countryside where no lights should be.... The strange lights were never explained."

Although another historian, Edwin Pears, dismisses the unusual sightings as "evidence of the superstition of the age," he admits that "they have to be taken into account, inasmuch as they affected the spirit of both besiegers and besieged."

In late May, Mehmet's advisers debated whether or not to continue the long siege. Thirty years earlier, Constantinople had successfully resisted a siege by Murat II, and perhaps, they thought, this effort would fare no better.

At about 1:30 in the morning of May 29, Mehmet launched a series of massive assaults on the walls. After four hours of fierce but inconclusive fighting, the commander of the Byzantines' Genoese allies was wounded, the Kerkoporta gate was breached—or possibly left open—and the tide of battle turned. By mid-morning, Constantinople belonged to the Ottoman Turks.

The link between volcanism and unusual atmospheric phenomena has been known for some time, but even today it is not fully understood. In 1784, Benjamin Franklin was one of the first to attribute unseasonably cold weather in Europe, and a strange "dry fog," as he called it, to an eruption that had taken place in Iceland the previous year. In 1815, the explosion of Tambora in Indonesia—the largest eruption of modern times—brought on the famous "year without a summer," a climate that may have inspired both Lord Byron's morose poem "Darkness" and Mary Shelley's novel Frankenstein.

But it was in 1883, when the Indonesian island of Krakatoa blew itself into the sky in the most notorious cataclysm of the 19th century, that researchers were first able to explain how a volcanic cloud could spread around the globe. They correctly attributed a hazy atmosphere, lurid sunsets and other optical effects to the eruption. Indeed, three months after Krakatoa, sunset afterglows were so brilliant and prolonged that fire companies in New York and Connecticut rushed out more than once to respond to what proved to be false alarms—an eerie parallel, Pang points out, to the flames and flickers recorded in Constantinople.

Today, Kuwae's underwater crater lies within the territory of the Republic of Vanuatu. For a long time, the explosion of the island had been known only in local legend. Now, geologists have calculated that Kuwae expelled between 32 and 39 cubic kilometers of molten rock—between 7.7 and 9.7 cubic miles of it—with a violence two million times that of the atomic bomb that destroyed Hiroshima. Only one other eruption in our era has surpassed Kuwae's in volume of output: Tambora blew 50 cubic kilometers of earth skyward in 1815. In contrast, even Krakatoa expelled a relatively modest 10 cubic kilometers in 1883; Mount Pinatubo belched out five in 1991 and Mount Saint Helens covered parts of the state of Washington with merely one-third of a cubic kilometer in 1980.

But it is not only the volume of expelled matter that might affect global climate. It is also how a volcano erupts that hastens or retards climatic effects. The more explosive a blast is—as opposed to what volcanologists call "effusive"—and the more prolonged it is, the more it can affect the atmosphere.

Just where a volcano is located—in which wind patterns—is also important. Global winds distribute clouds of material from tropical eruptions such as Kuwae, Krakatoa and Pinatubo widely into both northern and southern hemispheres, but material from eruptions at higher latitudes—such as Mount Saint Helens'—does not often drift as widely. And volcanic clouds with a high sulfur content are more likely to have climatic effects than their low-sulfur counterparts.

There is no longer any question that volcanic dust and other material—especially sulfur—can, under the right circumstances, block significant amounts of the sun's warmth from reaching the Earth. Some eruptions of the 19th and 20th centuries—not large on a geological scale—caused average temperatures at the Earth's surface to fall by 0.2 to 0.3 degrees Celsius (1/3°–1/2°F) worldwide over one to three years. Even such a small average drop can mean severe local temperature changes and, historically, such temperature downturns have had catastrophic effects on agriculture.

Pang has assembled strands of evidence from distant corners of the globe to support his thesis that Kuwae's eruption left "unmistakeable marks in world climate records." Many volcanic blasts have left their traces in—of all places—the polar ice sheets. Snow falling year after year near the poles sweeps volcanic fallout from the air and retains it as, over time, the snow is compressed into ice.

Today, the ice layers in core samples can be read like the pages of a book.

At the South Pole, glaciologists have found evidence in the ice cap for a large eruption in about 1450, give or take a few years. The breadth of the volcanic acid "spike" in this layer suggests that the eruption was likely to have affected the atmosphere for as long as three years. Another Antarctic ice core, called the Siple core, similarly displays a prominent acid peak beginning in 1453.

Evidence from the northern hemisphere's ice is less clear, although a core from the ice cap near Greenland shows a discernible, if relatively short, acid spike for 1453. The ice records thus do seem to point to a large eruption in the southern hemisphere which exerted a climatic effect on the northern hemisphere as well.

Pang has searched another archive of climate history, the annual growth rings of trees. Dendrochronologists—the term comes from the Greek words for "tree" and "time" and "knowledge"—know that thin annual rings speak of stress, as might be caused by cold weather, and that a season's accumulation of cells damaged by freezing creates what they call "frost rings." Drought has a similar effect, and such signs can be read not only in cores bored out of standing trees but also in wooden beams and other objects made of wood. At Windsor Castle, outside London, there is a portrait of Elizabeth Woodville—the mother of the young princes in the Tower—who lived from 1437 to 1492. It was painted on an oak panel from a tree cut down in the 1460's, and in that panel the tree rings for the years 1453 to 1455 are abnormally narrow.

Similarly, trees in France and Finland were stunted between 1453 and 1457, and even bristlecone pines in the western United States show frost damage for 1453. Agricultural evidence too seems to support Pang's case: Church records show that tithes of Swedish grain dropped to zero between 1453 and 1462. The German grape harvest was of poor quality from 1453 to 1456.

Cypress trees in China, too, grew narrow rings from 1453 to 1454. And according to Pang's own translations, the History of the Ming Dynasty records that "nonstop snow damaged the wheat crops" in central China in the spring of 1453, while heavy snow buried several provinces. "The Yellow Sea [between China and the Korean Peninsula]...was icebound more than 20 kilometers [13 miles] from shore. Tens of thousands of people and animals froze to death." South of the Yangtze River, an area with a mild climate like Florida's, "it snowed continuously for 40 days...and countless [people] died of cold and famine."

Kuwae's eruption seems to have taken place at about the right time to precipitate these meteorological calamities. In Vanuatu, oral tradition says that a tribal chief, Ti Tongoa Liseiriki, survived the eruption as a boy by hiding inside a drum. In later years, he led the resettlement of Tongoa, a small island near the vanished Kuwae. Archeological research has confirmed some aspects of the legend: Bones from the chief's grave, identified by the traditional boar's-tusk bracelets he wore, have been dated to approximately the same period, and radiocarbon dating of vegetation burned in the blast yields approximately synchronous dates.

Did the volcanic cataclysm of Kuwae really produce the portents that haunted the siege and capture of Constantinople halfway around the world?

"We know that volcanism can cause strange effects in the atmosphere, and that the volcano erupted around the same time that Constantinople fell," says Haraldur Sigurdsson, a volcanologist at the University of Rhode Island in Kingston. He finds the evidence intriguing, but he remains skeptical because it is circumstantial.

"You try to build a case from as many types of climatic records as possible," notes Peter Kuniholm, a dendrochronologist at Cornell University in Ithaca, New York. "Only when a number of separate lines of investigation agree on the same thing are we safe in positing a true climatic effect or change." Although Kuniholm's specialty is ancient trees in the Mediterranean region, he has so far sampled only Turkish trees that grew at too low an altitude to suffer possible volcanic effects. Trees growing at higher elevations, he says, are more likely to have recorded temperature changes.

Kuwae's role in the capture of Constantinople in 1453 has yet to be definitively proved. But if Pang is correct, the pro-Byzantine Georgian writer who lamented poetically that on the day Constantinople was taken, "the sun was darkened," may have been accurate in a way he could never have imagined.Freelance science writer Lynn Teo Simarski, based in Alexandria, Virginia, has written about the climatic effects of volcanism for the American Geophysical Union, and is a frequent contributor to Aramco World.